Half-bridge circuits are used quite generally to generate an AC signal from a DC voltage that is supplied on the input side. When connected up in an appropriate manner, half-bridge circuits of this type may also be used to generate polyphase AC signals, in particular also three-phase current signals. The output-side (if appropriate polyphase) AC signal is then supplied to a motor, for example.
A multiplicity of variations of half-bridge circuits are known in accordance with the prior art. Half-bridge circuits on which the invention is based are essentially based on two power transistors and two diodes which are usually respectively integrated on a single chip. In this case, the power transistors used are preferably so-called insulated gate bipolar transistors (IGBTs) on account of their low control powers and their small forward resistances on the collector-emitter path. Field effect power transistors, preferably MOSFETs, are also used for relatively low powers.
In this case, the forward current paths, i.e. the source-drain paths or collector-emitter paths, of the power transistors are connected in series on the input side. When the polarity is reversed, a respective diode is connected in parallel with the two power transistors in this case. In concrete terms, this means that, when using npn IGBTs, the collector electrode of an IGBT is respectively connected to the corresponding anode of the parallel-connected diode and the emitter electrode of the IGBT is connected to the cathode of the corresponding parallel-connected diode. The phase output, at which the AC voltage can be tapped off, is located at the node between the two series-connected power transistors.
In the event of a short circuit, it is desirable to negatively feed back the drive signal of the power transistors as much as possible in order to limit the turn-off speed. In the case of normal operation, however, too much negative feedback is associated with increased losses, which is undesirable.
In order to generate such negative feedback of the drive signal, in accordance with the prior art, said drive signal is therefore not applied directly to the corresponding control inputs (in particular the gate and source or the gate and emitter in the abovementioned embodiment variants) of the discrete power transistors (initially assumed to be ideal) but rather is applied via respective inductances arranged in the forward current paths of the two power transistors. The control inputs which are arranged in the forward current paths downstream of the inductances are generally referred to as auxiliary electrodes, i.e., for example, as the auxiliary source electrode or auxiliary emitter. In accordance with the prior art regarding inductive negative feedback the values selected for the inductances represent a compromise between limiting switching losses during normal operation and limiting the turnoff speed of the corresponding power (switching) transistor in the event of a short circuit.
In practice, use is made of the fact that, in the half-bridge circuit described above, each line section, i.e. each conductor track and each connection realized with the aid of bonding wires or the like, between the individual discrete components (initially assumed to be ideal), namely the power transistors and the diodes, represents a (line/leakage) inductance, it being possible to predetermine the magnitude of each of these inductances within a wide range, if appropriate with regard to the concrete application. Arranging the corresponding (auxiliary) electrodes (for example the auxiliary source electrode or auxiliary emitter) in a suitable manner in the respective line sections that lead away from the electrodes (for example the source or emitter) thus makes it possible to establish the magnitude of the inductances that are required for the negative feedback and the level of the respective negative feedback.
In order to elucidate the facts specified above, reference is made by way of example to a concrete embodiment of a half-bridge circuit (in accordance with the prior art) outlined in FIG. 3 of the drawings.
FIG. 3 shows an equivalent circuit diagram of a half-bridge circuit (in accordance with the prior art) based on two IGBTs and two diodes. In this case, in addition to the circuit symbols symbolizing these (ideal) components, the equivalent circuit diagram also reveals the (parasitic) leakage and/or line inductances that are implicitly or intentionally present in each lead and are identified by corresponding discrete circuit symbols.
The half-bridge circuit shown in FIG. 3 has two input terminals 15 and 16—via which an input DC voltage VE can be supplied—and a phase output P, at which the AC voltage generated can be tapped off. This phase output P represents, for example, a phase of a three-phase network.
The first input terminal 15 is connected to the node 7 via the (leakage) inductance L14. The node 7 is connected, on the one hand, to the collector C1 of the first insulated gate bipolar transistor 1 via the (leakage) inductance L12 and, on the other hand, to the cathode K1 of the first diode D1 via the (leakage) inductance L13. The emitter E1 of the first insulated gate bipolar transistor 1 is led to the node 8. Furthermore, the anode A1 of the diode D1 is connected to the node 9 and the latter is in turn connected to the node 8 via the (leakage) inductance L11. The node 8 is in turn led to the node 10 via the (leakage) inductance L24. In accordance with the prior art, this node 10 forms, on the one hand, the terminal point for the phase output P, the (line/leakage) inductances of which are symbolized, in the figure of the drawing in question, by corresponding circuit symbols identified by the reference symbols L15 and L16, and, on the other hand, the terminal point for the second series-connected power/switching transistor (IGBT 2).
Taking into account line and/or leakage inductances, the node 10 establishes a connection, on the one hand, to the collector terminal C2 of the second insulated gate bipolar transistor 2 via the (leakage) inductance L22 and, on the other hand, to the cathode K2 of the second diode D2 via the (leakage) inductance L23. The emitter E2 of the second insulated gate bipolar transistor 2 is led to the node 11. The anode A2 of the second diode D2 is likewise connected to the node 11 via the (leakage) inductance L21. A line that is represented by the inductance L27 in turn leads away from the node 11 to the node 12 and from there onward to the input terminal 16.
In order to obtain the abovementioned desired negative feedback, the drive signals for the two insulated gate bipolar transistors 1, 2 are not switched directly between the respective control terminals gate G1 and G2 and emitter E1 and E2 of the insulated gate bipolar transistors 1 and 2 but rather the drive signals are injected further away from these control terminals gate G1 and G2 and emitter E1 and E2 on the existing connection paths between the emitter E1 of the first insulated gate bipolar transistor 1 and the collector C2 of the second insulated gate bipolar transistor 2 and between the emitter E2 of the second insulated gate bipolar transistor 2 and the input terminal 16. The corresponding terminal points which are also referred to as auxiliary emitters in the jargon are identified by the reference symbols HE1 (auxiliary emitter of the IGBT 1) and HE2 (auxiliary emitter of the IGBT 2) in FIG. 3. Accordingly, the auxiliary emitter HE1 of the first insulated gate bipolar transistor 1 is situated directly at the node 10 to which the phase output P is connected and the auxiliary emitter HE2 of the second insulated gate bipolar transistor 2 is situated at the node 12 that in turn establishes a connection to the input terminal 16.
FIG. 3 shows the commutation of a DC voltage VE (which is supplied on the input side) during normal operation of the half-bridge circuit. Accordingly, when the first insulated gate bipolar transistor 1 is turned on, a current flows along the current path (identified by the reference symbol 13 in the figure of the drawing) to the phase output P on account of the positive voltage + applied to the first input terminal 15: in accordance with FIG. 3, the current path runs to the phase output P via the inductance L14, the node 7, the inductance L12, the collector-emitter path C1-E1 of the insulated gate bipolar transistor 1, the node 8, the inductance L24 and the node 10.
In accordance with Lenz's law, when the first insulated gate bipolar transistor 1 is turned off, the current flowing to the phase output P will initially continue to flow. However, since the current path 13 to the positive terminal 15 has been interrupted by the turned-off IGBT 1, the current commutates to the current path identified by the reference symbol 14. Accordingly, when the first insulated gate bipolar transistor 1 is off, the second input terminal 16 (negative voltage pole−of the input DC voltage VE) is connected to the phase output P via the node 12, the inductance L27, the node 11, the inductance L21, the diode D2 connected in the forward direction, the inductance L23 and the node 10.
During the normal commutation process, the current intensity in the negative feedback inductances L24 and L27 therefore changes. Negative feedback that is associated with undesirable switching losses therefore takes place.
The same applies to the case of a phase short circuit. Assume, by way of example, that there is a short circuit between the phase output P and the second input terminal 16. In this case, when the IGBT 1 is turned on, a current will flow from the positive input terminal 15 to the negative input terminal 16 via the inductance L14, the node 7, the inductance L12, the collector-emitter path C1-E1 of the IGBT 1, the inductance L24, the node 10, the inductance L15 and the short-circuit path.
When the first insulated gate bipolar transistor 1 is turned off, the current intensity in the inductance L24 changes. The negative feedback that is desired in this case in order to limit the turn-off speed of the first insulated gate bipolar transistor 1 therefore takes place.
Although the abovementioned embodiment has fundamentally proven successful, the negative feedback of the driving of the two switching transistors that is needed to limit the turn-off speed of the latter in the event of a short circuit leads to switching losses during normal operation of the half-bridge circuit.